The field of the present invention generally relates to oculars or eyepieces for a riflescope or spotting scope or other types of telescopic optical systems.
Telescopic sights for rifles typically include an ocular or eyepiece and objective lenses positioned at opposite ends of a tubular housing. Conventional variable optical power telescopic sights include an erector lens or lens assembly positioned medially of the eyepiece and objective lenses movable along a central longitudinal axis of the housing for adjustment of the optical power.
Most oculars use lenses that have only spherical surfaces. To reduce aberrations, ordinarily, the number of lenses must be increased to achieve a desired aberration correction. This increase in the number of lenses increases both the weight and cost of the ocular. Efforts have been made by using glass lenses with aspheric surfaces to reduce aberrations. For example the article by John Hall and Jay Vizgaitis, “Aspheric Diet”, Photonics Spectra (December 2000) describes a riflescope eyepiece in which two spherical lens elements are replaced with a single aspheric glass lens within an otherwise all spherical lens assembly.
The present inventors have recognized that aspheric glass lenses are expensive to manufacture and thus are not cost-effective for mass-production components in a riflescope, but that it would be desirable if a riflescope ocular could be economically adapted to accommodate aspheric lenses.